Dresden 2006 – wissenschaftliches Programm

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CPP: Chemische Physik und Polymerphysik

CPP 4: New Experimental Techniques

CPP 4.8: Vortrag

Montag, 27. März 2006, 16:15–16:30, ZEU Lich

Periodic NMR excitation in very thin slices: Interplay of diffusion and relaxation — •Achim Gädke and Nikolaus Nestle — TU Darmstadt, Institute of condensed matter physics, Hochschulstr. 6, D-64289 Darmstadt, Germany

NMR experiments involving thin excited slices are of interest both for conventional micro-MRI and for mechanically detected magnetic resonance. Especially in the latter case, the thickness of the excited slices may be well below 1 µm. In samples containing fast-diffusing liquid components such as water, the diffusive exchange of spin magnetization between the excited slice and its surroundings on the time scale of the NMR experiments plays a major role. Some time ago, we have studied the influence of the diffusion balance on the apparent longitudinal relaxation behaviour measured in a saturation-recovery experiment. This study suggests that in a 1 µm slice with a Gaussian profile, the apparent longitudinal relaxation time of water may be below 1 ms. In this experiment, full spin relaxation was possible between two excitation cycles. In micro-MRI or in mechanically detected NMR, by contrast, the excitation is periodic. In this case, the diffusive magnetization balance between the excited slice and its surroundings needs considering excited magnetization from previous excitation cycles, too. This diffusion balance will also depend on the presence of diffusion barriers and relaxation sinks outside the excited slice. Results from both simulation studies and experiments concerning these two effects are presented, and their implications micro-MRI and mechanically detected NMR are discussed.